It is important in many applications to identify ions within a sample. For example, a manufacturing process may be monitored by identifying the ions present at various stages of the manufacturing process and by determining the concentration of the ions. If necessary, the manufacturing process can then be adjusted to bring the concentration of the ions into the proper balance.
One industry in which ion concentration is closely monitored is the pulp and paper industry. During the chemical pulping of wood to obtain cellulose for papermaking, for example, inorganic chemicals are mixed with wood chips to separate lignin from the wood fibers. The lignin is then removed by cooking the wood chips in an aqueous solution of sodium hydroxide (NaOH) and white liquor. The white liquor includes Na.sub.2 S and smaller amounts of Na.sub.2 CO.sub.3, Na.sub.2 SO.sub.4, Na.sub.2 S.sub.2 O.sub.3 and Na.sub.2 SO.sub.3. Once cooked, the spent liquor, i.e., black liquor containing both organic and inorganic anions, is concentrated and burned in a recovery furnace to obtain a smelt of Na.sub.2 CO.sub.3 and Na.sub.2 S. The molten sodium salts are dissolved to form green liquor which is then reacted with calcium hydroxide (Ca(OH).sub.2) to regenerate the white liquor.
In addition to monitoring the temperature/time profile of chip digestion, the liquor composition must be carefully controlled during all phases of the kraft pulping process in order to properly separate lignin from the wood fibers. In this regard, the composition of the liquors obtained from the combustion of the black liquor and the regeneration of the white liquor will reveal whether targeted performance schedules have been met and, in turn, will determine the quality of the resulting cellulose. The liquors of the kraft pulping process include a number of anions, namely, hydroxide, sulfide, carbonate, sulfate, chloride, thiosulfate and sulfite anions, which are critical to the quality of the pulping. As a result, a significant portion of mill time is consumed by the monitoring of the concentrations of these anions to determine process performance. Historically, this anion analysis has been performed off-line by various gravimetric and titrimetric procedures. See D. R. Salomon et al. "Applications of Capillary Ion Electrophoresis in the Pulp and Paper Industry", Journal of Chromatography, Vol. 602, pp. 219-25 (1992).
In addition to the gravimetric and titrimetric procedures utilized in the kraft pulping industry, a number of other ion analysis methods have been developed. For example, a number of electrophoresis methods have been developed for identifying and analyzing ions. Electrophoresis involves the migration of electrically charged particles, such as ions, in solution or suspension in the presence of an applied electric field. For a given set of solution conditions, the velocity with which an ion moves divided by the magnitude of an electric field is a characteristic number called the electrophoretic mobility. The electrophoretic mobility is directly proportional to the magnitude of the charge on the particle and is inversely proportional to the size of the particle. By separating ions from each other in the electric field on the basis of charge or size, the different ions can be identified and measured.
Capillary electrophoresis (CE) and, more particularly, capillary zone electrophoresis (CZE) are variations of standard electrophoresis techniques. In capillary zone electrophoresis, a capillary of a nonconducting material, such as fused silica, is filled with a buffer solution and is immersed in buffer reservoirs at both ends. The sample to be analyzed, i.e., the analyte, is applied to the head of the capillary in a narrow band having a width much less than the length of the capillary tube and at concentrations too low to affect the conductivity of the buffer solution. By applying a relatively large electric field across the length of the capillary tube, such as 10,000-30,000 volts, the various species within the sample separate into zones based upon their respective electrophoretic mobilities and migrate through the capillary tube. The moving analyte bands or zones are detected based upon ultraviolet light absorption or fluorescence emission, or an electrochemical property of the moving bands. In either instance, the capillary zone electrophoresis technique obtains a value relating the measured property to the different analyte zones. The measured property of each analyte zone defines, at least in part, various parameters relating to the respective ion, such as the concentration of the respective ion.
Conventional capillary electrophoresis techniques include a detector such as an ultraviolet absorption detector for both illuminating the moving analyte sample and for detecting the amount of light absorbed by the moving analyte zones. Based upon the data collected by the ultraviolet absorption detector, a measure of the degree of transparency of the sample and, in turn, a measure of the concentration of the respective ions is obtained. Typically, however, the data collected by the ultraviolet absorption detector must be analyzed by the system operator in order to determine the presence and/or the concentration of the various ions. As will be apparent to those skilled in the art, the data analysis can be quite time consuming and labor intensive. It is also relatively difficult to compare the capillary electrophoresis analysis results of samples obtained at two different times since there is no common base line.
Conventional instruments for performing capillary electrophoresis typically include several stand-alone hardware modules that must be properly interconnected in order to receive the data collected by the detector and to generate corresponding electropherograms. In addition to the relatively large cost of the hardware modules and the difficulties encountered in properly interconnecting the hardware modules, the stand-alone hardware modules of conventional instruments for performing capillary electrophoresis are generally configured to receive data collected by one specific type of detector, such as an absorption detector, a fluorescence detector or an electrochemical detector. As such, the stand-alone hardware modules of conventional instruments for performing capillary electrophoresis cannot typically receive and process data collected by detectors other than the one specific type of detector for which they are designed.
As with conventional gravimetric and titrimetric methods, capillary electrophoresis and, in particular, capillary zone electrophoresis is typically performed off-line. In this respect, a sample is obtained from one or more stages of a manufacturing process. These samples are then analyzed by capillary electrophoresis as the manufacturing process continues. Based upon the results of the capillary electrophoresis analysis, such as based upon the presence and/or the concentration of the various ions within the sample, the manufacturing process can be adjusted to bring the concentrations of the respective ions within a desired range. The time delays inherent in this off-line analysis, including the time required by the system operator to analyze the data, can create problems if the analysis indicates that the manufacturing process was operating outside of specifications. In this event, the resulting product produced during the time required to conduct the capillary electrophoresis analysis may have to be discarded or reprocessed.